Silicon oxide-based negative electrode material and method of manufacturing the same

ABSTRACT

[Problem] Provided is a silicon oxide-based negative electrode material capable of avoiding, as much as possible, decreased battery performance resulting from a heterogeneous distribution of a Li concentration. [Solution] Provided is a powder having an average composition of SiLixOy wherein 0.05&lt;x&lt;y&lt;1.2 and a mean particle size of 1 μm or more. Further, 10 particles randomly selected from particles of the powder each satisfy 0.8&lt;L1/L2&lt;1.2 with the standard deviation of L2 being 0.1 or less, L1 being a Li concentration at a depth of 50 nm from an outermost surface of each of the 10 particles, and L2 being a Li concentration at a depth of 400 nm from the outermost surface.

TECHNICAL FIELD

The present invention relates to a silicon oxide-based negativeelectrode material for use in forming a negative electrode of a Li-ionsecondary battery and a method of manufacturing the same. Morespecifically, the present invention relates to a method of manufacturinga silicon oxide-based negative electrode material in which Li is dopedto cancel irreversible capacity.

BACKGROUND ART

Silicon oxide (SiO_(X)) is known to be a negative electrode material fora Li-ion secondary battery having a large electric capacity andexcellent lifetime properties. Such a silicon oxide-based negativeelectrode material is processed into a film-like negative electrode bymixing a silicon oxide powder, an electrically conductive auxiliaryagent, and a binder to form a slurry, and applying the slurry onto acurrent collector of a copper foil and the like. The above silicon oxidepowder may be obtained by, for example, heating a mixture of silicondioxide and silicon to generate a silicon-monoxide gas, cooling thesilicon-monoxide gas to produce a deposit, and then finely pulverizingthe deposit. A silicon oxide powder as manufactured by the abovedeposition method is known to be largely amorphous, leading to reducedvolume change during charge and discharge, which in turn can increasecycle properties.

Nonetheless, such a silicon oxide-based negative electrode materialsuffers from a characteristically low initial efficiency. This may beevident by a phenomenon in which a Li compound responsible forirreversible capacity that does not contribute to charge and dischargeis produced during the initial change, resulting in significantlydecreased initial discharge capacity. As an approach of solving thisproblem, known is a Li-doping method in which Li ions are added to asilicon oxide powder.

For example, Patent Document 1 proposes a solid phase method of heatingand calcining a mixture of a silicon oxide powder and a metal Li powderor a mixture of a metal Li powder and a Li compound powder under aninert gas atmosphere or under reduced pressure. Further, Patent Document2 proposes a gas phase method of generating a SiO gas and a Li gasseparately, then mixing both gases to produce a gas mixture, and coolingthe gas mixture to allow for collection. In either method, theproduction of a Li compound responsible for irreversible capacity duringthe initial charge and discharge can be reduced by pre-forming a Licompound responsible for irreversible capacity that does not contributeto charge and discharge. As would be expected, the initial efficiencymay be improved. This may be referred to as an “irreversible capacitycancelling process.”

However, a silicon oxide-based negative electrode material subjected tothe irreversible capacity cancelling process by means of Li doping maycompromise battery performance due to heterogeneous Li doping. This isperceived as an issue to be solved.

For example, in the solid phase method (deposition method) as describedin Patent Document 1, Li is doped via a reaction at the surface ofparticles of a powder in which Li ions are doped in particles of asilicon oxide powder through their surfaces during calcination. Thistends to result in a heterogeneous distribution of a Li concentration inthe inside of particles of the powder, and in particular, tends toresult in a higher Li concentration at the surface of the particles.Further, a varied composition of the powder may also be responsible fora heterogeneous distribution of a Li concentration in the inside ofparticles of the powder, in particular, a heterogeneous distribution ofa Li concentration at the surface of the particles.

Meanwhile, in the gas phase method (deposition method) as described inPatent Document 2, homogenous mixing of a SiO gas and a Li gas andcontrol of temperature and partial pressure are very difficult toachieve. This may inevitably result in a heterogeneous distribution of aLi concentration in the gas mixture, and thus a heterogeneousdistribution of a Li concentration in the resulting deposit. Further,the deposit will be pulverized to obtain a powder of a negativeelectrode material. Therefore, the heterogeneous distribution of a Liconcentration in the deposit will be responsible for a heterogeneousdistribution of a Li concentration among particles of the powder for anegative electrode material.

When a heterogeneous distribution of a Li concentration occurs either inthe inside of particles of a powder or among the particles, a highlyreactive Li-rich phase such as a LiSi alloy and the like may be formedat a portion having a higher Li concentration. The Li-rich phase, whichmay react with a binder and a solvent during the aforementioned processof manufacturing an electrode, is responsible for deteriorated batteryperformance.

CITATION LIST

Patent Document 1: Japanese Patent No. 4702510

Patent Document 2: Japanese Patent No. 3852579

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a silicon oxide-basednegative electrode material capable of avoiding, as much as possible,decreased battery performance resulting from a heterogeneousdistribution of a Li concentration, and a method of manufacturing thesame.

Solution to Problem

As described above, a significant issue of a silicon oxide-basednegative electrode material doped with Li resides in a heterogeneousdistribution of a Li concentration in a silicon oxide powder. Moreover,the heterogeneous distribution of a Li concentration in a silicon oxidepowder may be represented in terms of a heterogeneous distribution of aLi concentration in particles of the powder and a heterogeneousdistribution of a Li concentration among particles of the powder asdescribed above.

The present inventors believed that the case of two differentheterogeneous distributions of a Li concentration in a silicon oxidepowder, i.e., a heterogeneous distribution of a Li concentration inparticles of the powder and a heterogeneous distribution of a Liconcentration among particles of the powder, needs to be resolved inorder to avoid decreased battery performance resulting from aheterogeneous distribution of a Li concentration. Then the presentinventors focused on developing a method of manufacturing a Li-dopedsilicon oxide powder. Particular attention was paid for a gas phasemethod (deposition method) such as one described in Patent Document 2.

In a gas phase method (deposition method), a mixture of SiO and Li ismanufactured from a SiO gas and a Li gas. The easiest method couldinclude simply mixing a source of a SiO gas with a source of a Li gas toobtain a mixture and heating the mixture. However, a SiO gas and a Ligas will not be generated simultaneously according to this method. Onlya Li gas, which has a higher vapor pressure, will be preferentiallygenerated. For this reason, a material in which SiO and Li arehomogeneously mixed cannot be obtained.

Another gas phase method (deposition method) could include heating acomposite compound of Li and Si represented by lithium silicate such asLi₂Si₂O₅ under reduced pressure to generate a SiO gas and a Li gassimultaneously. However, lithium silicate alone will not generate a gaseven when heated under reduced pressure. In contrast, it was found thateven for such lithium silicate, simultaneous generation of a SiO gas anda Li gas can be achieved when heated under reduced pressure in theco-presence of Si, particularly in the co-presence of elemental Si,leading to the generation of a gas in which SiO and Li are homogeneouslymixed. It is also found that the above gas can be deposited to obtain amaterial in which SiO and Li are homogeneously mixed.

The material in which SiO and Li are homogeneously mixed can be thenpulverized to obtain a Li-containing SiO powder. The resultingLi-containing SiO powder can be used to produce a negative electrodematerial in which a heterogeneous distribution of a Li concentrationamong particles of the powder and a heterogeneous distribution of a Liconcentration in an individual particle of the powder are both resolved.Simultaneous generation of a SiO gas and a Li gas from lithium silicatein the co-presence of elemental Si can likely be explained if lithiumsilicate is reduced by elemental Si.

The present invention is made based on the above findings. A siliconoxide-based negative electrode material according to an embodiment ofthe present invention is technologically characterized in that itcomprises a powder having an average composition of SiLi_(x)O_(y)wherein 0.05<x<y<1.2 and a mean particle size of 1 μm or more,

in which 10 particles randomly selected from particles of the powdereach satisfy 0.8<L1/L2<1.2 with the standard deviation of L2 being 0.1or less, L1 being a Li concentration at a depth of 50 nm from anoutermost surface of each of the 10 particles, and L2 being a Liconcentration at a depth of 400 nm from the outermost surface.

In the silicon oxide-based negative electrode material according to anembodiment of the present invention, L1 as a Li concentration at a depthof 50 nm from an outermost surface of each particle means a Liconcentration at the surface of a particle, and L2 as a Li concentrationat a depth of 400 nm from the outermost surface means a Li concentrationin the inside of the particle. The expression “randomly selected 10particles satisfy 0.8<L1/L2<1.2” means that the Li concentration ishomogeneous from the surface through to the inside of a particle, i.e.,the heterogeneous distribution of a Li concentration in a particle isresolved. Further, the expression “the standard deviation of L2 amongthe 10 particles is 0.1 or less” means that the heterogeneousdistribution of a Li concentration among the 10 particles is resolved.Therefore, decreased battery performance resulting from a heterogeneousdistribution of a Li concentration can be avoided effectively.

When even one of the randomly selected 10 particles does not satisfy0.8<L1/L2<1.2, a Li-rich phase may appear due to the heterogeneousdistribution of a Li concentration in a particle, and a powder havinglow reactivity and high battery performance may not be obtained. Whenthe standard deviation of L2 is greater than 0.1, a Li-rich phase mayappear due to a heterogeneous distribution of a Li concentration amongparticles, and a powder having low reactivity and high batteryperformance may not be obtained.

For the Li concentrations L1 and L2, the cross-section of a particle ofthe powder is observed under TEM, and then EELS measurements areperformed at a respective predetermined depth for a region of, forexample, 20 nm in the longitudinal direction and 400 nm in thetransverse direction to obtain a ratio of a Li spectral intensity to aSi spectral intensity, from which the Li concentrations can berelatively determined.

When x is too small in the average composition SiLi_(x)O_(y) of apowder, the addition effects of Li cannot be fully obtained. When x isgreater than or equal to y, a LiSi alloy may be formed, resulting inincreased reactivity of a powder. When y is too large, the charge anddischarge capacity of a powder is decreased. For these reasons, therange 0.05<x<y<1.2 is selected. Each elemental ratio can be measured bythe ICP emission spectrometry method and the infrared absorption method.

L2 as a Li concentration in the inside of a particle is defined as a Liconcentration at a depth of 400 nm from the outermost surface. If themean particle size of the particles of the powder is less than 1 μm, L2may not be able to correctly represent a Li concentration in the insideof the particle and may result in a less reliable value of L1/L2.Therefore, 1 μm or more is selected for the mean particle size ofparticles of the powder. There is no particular limitation for the upperlimit of the mean particle size, but it is preferably 20 μm or lessbecause a larger mean particle size may complicate an applicationprocess to an electrode, and further may tend to result in a crack anddeteriorated performance due to expansion and contraction during chargeand discharge.

A method of manufacturing the silicon oxide-based negative electrodematerial according to an embodiment of the present invention involvesheating a raw material containing Si, O, and Li to generate a SiO gasand a Li gas simultaneously from the raw material; and cooling thesegasses on the same surface to allow for collection.

In the method of manufacturing the silicon oxide-based negativeelectrode material according to an embodiment of the present invention,the SiO gas and the Li gas simultaneously generated are cooled on andcollected from the same surface as the SiO gas and the Li gas aresimultaneously generated from the raw material. This allows a deposit tobe produced in which SiO and Li are homogeneously mixed. Afterpulverization and powderization of the deposit, a powder can be obtainedin which a heterogeneous distribution of a Li concentration amongparticles of the powder and a heterogeneous distribution of a Liconcentration in individual particles are both resolved.

Specifically, the raw material containing Si, O, and Li represents aSi.lithium silicate-containing raw material in which a portion of Si ispresent as elemental Si, and Li is present as lithium silicate. Use ofthe above raw material enables simultaneous generation of a SiO gas anda Li gas from lithium silicate by heating that lithium silicate in theco-presence of elemental Si.

The Si.lithium silicate-containing raw material is typically a mixtureof elemental Si and lithium silicate or a mixture of elemental Si,lithium silicate, and a Si oxide. The Si oxide is contained foradjusting the O content and the like and may be SiO_(X) (0<X≤2) such asSiO and SiO₂. Lithium silicate is represented by the general formulaxLi₂O._(y)SiO₂, and specifically may be Li₂Si₂O₅ (x=1, y=2), Li₂SiO₃(x=1, y=1), Li₄SiO₄ (x=2, y=1), Li₆Si₂O₇ (x=3, y=2), or the like.

In the raw material containing Si, O, and Li, a material which produceslithium silicate upon heating may be used instead of lithium silicate.Specifically, it may be a material including one or both of LiOH andLi₂CO₃ and elemental Si, which can be heated and calcined as a primaryraw material. When LiOH or Li₂CO₃ is heated and calcined in theco-presence of elemental Si, lithium silicate is generated whileundesired elements are also removed as gas components to yield aSi.lithium silicate-containing raw material including elemental Si andlithium silicate. Heating the above Si.lithium silicate-containing rawmaterial as a secondary raw material will generate a SiO gas and a Ligas simultaneously. In addition, a material including Li₂O, elementalSi, and a Si oxide (SiO_(X); 0<X≤2); a material including an organiclithium compound and the like instead of Li₂CO₃; and the like may beused as a primary material. However, a material including one or both ofLiOH and Li₂CO₃ in addition to elemental Si is particularly preferred inview of cost and easy handling of a raw material. The primary rawmaterial may also include a Si oxide (SiO_(X); 0<X≤2) for adjusting theO content and the like in a similar manner as the secondary rawmaterial, i.e., the above Si.lithium silicate-containing raw material.

Here, a reaction for forming lithium silicate may be performedimmediately before a reaction for simultaneously generating a SiO gasand a Li gas. That is, a secondary raw material may be heatedcontinuously after the primary raw material is heated and calcined toform the secondary raw material in the same reaction vessel.Alternatively, the primary raw material may be heated and calcined inadvance to form a secondary raw material. When the primary raw materialis heated and calcined under reduced pressure, impurity elements may bemore easily separated. When the primary raw material is heated andcalcined in advance, heating and calcination is preferably performedunder an inert gas atmosphere or under reduced pressure.

The average composition of a raw material containing Si, O, and Li maybe expressed by SiLi_(x)O_(y), in which 0.05<x<y<1.2 is preferred, andin particular, 0.05<x<0.7 is preferred for x, and 0.9<y<1.1 is preferredfor y. The contents of Li, Si, and O in the raw material are adjustedwithin these ranges so as to obtain a desired element ratio.

When x is too small, the addition effects of Li may not fully beobtained. On the other hand, a larger value of x may increase the yieldof a Li gas, resulting in the formation of a highly reactive Li-richphase. When a value of y is too small or too large, the raw material mayleave an increased amount of residue, and in addition, a larger amountof a Li gas may be generated due to an altered composition ratio of Li,resulting in the formation of a Li-rich phase.

A material (deposit) collected after cooling on the same surface may beprocessed, i.e., pulverized so as to have a predetermined particle sizeto obtain a powder for a negative electrode material. There is noparticular limitation for a method of pulverization, but a method ispreferred in which necessary measures are taken to prevent contaminationof metal impurities, and specifically a method is preferred in which anon-metal material such as ceramic is used for a portion to be incontact with a powder.

For a powder of a negative electrode material, the surface of a particleof the powder may be partially or entirely coated with an electricallyconductive carbon film. The coating of an electrically conductive carbonfilm can reduce surface resistance and improve battery properties. Theelectrically conductive carbon film for use herein may be obtained by,for example, a thermal CVD reaction with a hydrocarbon gas. However,there is no particular limitation for a method of achieving this.

Advantageous Effects of Invention

The silicon oxide-based negative electrode material according to anembodiment of the present invention can effectively avoid decreasedbattery performance due to a heterogeneous distribution of a Liconcentration by resolving a heterogeneous distribution of a Liconcentration in particles of a powder for a negative electrode materialand a heterogeneous distribution of a Li concentration among particlesof the powder. This can significantly improve battery performance.

Further, the method of manufacturing the silicon oxide-based negativeelectrode material according to an embodiment of the present inventioncan produce a powder for a negative electrode material in which aheterogeneous distribution of a Li concentration in particles of thepowder and a heterogeneous distribution of a Li concentration amongparticles of the powder are both resolved, and thus can effectivelyavoid decreased battery performance due to a heterogeneous distributionof a Li concentration. This can significantly improve batteryperformance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an image of a cross-section of a particle of a powderobserved under a TEM, the powder pertaining to the silicon oxide-basednegative electrode material according to an embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENT

Below, the embodiments of the present invention will be described. Thesilicon oxide-based negative electrode material according to anembodiment of the present invention may typically be manufactured by thefollowing methods.

First, a Si powder is mixed with, for example, a Li₂Si₂O₅ powder as alithium-silicate powder to prepare a raw material containing Si, O, andLi. A SiO₂ powder is mixed to adjust the O content if necessary. Themixing ratio of the powders is adjusted so as to obtain the desiredelement ratio (Li:Si:O) of Li, Si, and O (for example, 1:0.4:1) within arange where the average composition SiLi_(x)O_(y)of the powder mixturesatisfies 0.05<x<y<1.2.

Next, the above powder mixture as a raw material is changed into areaction vessel, and heated under reduced pressure to generate a gasfrom, in particular, lithium silicate contained in the mixed rawmaterial. In the above gas-generating reaction, a SiO gas and a Li gasare generated simultaneously. With reference to chemical formulae, thereaction may be expressed generally by formula (1), or by formula (2)when lithium silicate is Li₂Si₂O₅. As described above, lithium silicatemay be represented by the general formula of _(x)Li₂O._(y)SiO₂.

[Formulae 1]

(x+y)Si+(xLi₂O.ySiO₂)→(x+2y)SiO ↑+2xLi ↑  (1)

3Si+Li₂Si₂O₅→5SiO ↑+2Li ↑  (2)

As shown in formula (1) and formula (2), a SiO gas and a Li gas aregenerated simultaneously from lithium silicate by heating in theco-presence of elemental Si. This reaction is believed to be a reductivereaction by Si.

While allowing the gases to be generated from the raw material in thereaction vessel, the generated gases are cooled and deposited on asurface of a vapor deposition platform arranged at an upper portionwithin the reaction vessel. After the end of the reaction, a deposit iscollected from the surface of the vapor deposition platform. Thecollected deposit, which is a Li-containing silicon oxide material, ispulverized to obtain a powder for a negative electrode material having apredetermined particle size.

The SiO gas and the Li gas are simultaneously generated from the rawmaterial within the reaction vessel. This allows for the production of agas mixture of the two having their concentrations homogeneouslydistributed. Therefore, a deposit obtained by cooling the above gasmixture on the same surface of a vapor deposition platform will alsohave homogeneously distributed concentrations. Therefore, a powderobtained by pulverizing the above deposit will have both a homogeneousdistribution of a Li concentration among particles of the powder and ahomogeneous distribution of a Li concentration in individual particlesof the powder. When the above powder is used as a powder for a negativeelectrode material, development of a Li-rich phase can be prevented,leading to decreased reactivity and improved battery performance.

In another embodiment, a Si powder is mixed with a LiOH powder. A SiO₂powder is mixed to adjust the O content if necessary. The resultingpowder mixture as a primary raw material is changed into a reactionvessel and heated and calcined under an Ar atmosphere. With reference tochemical formulae, a reaction in which LiOH is heated in the co-presenceof elemental Si may be expressed by the first part of formula (3).

[Formulae 2]

4Si+4LiOH→3Si+Li₄SiO₄+2H₂ ↑ 3Si+Li₄SiO₄→4SiO ↑+4Li ↑  (3)

4Si+2Li₂CO₃→3Si+Li₄SiO₄+2CO ↑ 3Si+Li₄SiO₄→4SiO ↑+4Li ↑  (4)

As shown in the first part of formula (3), heating and calcining LiOH inthe co-presence of elemental Si generates lithium silicate (Li₄SiO₄)while an undesired element H is removed as a gas component. Theresulting calcined material is a mixture of lithium silicate (Li₄SiO4)and residual element Si. This corresponds to the raw material containingSi, O, and Li used in the aforementioned embodiment.

Then, heating of the resulting calcined material as a secondary rawmaterial is continued under reduced pressure. Heating of lithiumsilicate (Li₄SiO₄) contained in the secondary raw material in theco-presence of elemental Si then generates a Si gas and a Li gassimultaneously from that lithium silicate (Li₄SiO₄) as shown in thelatter part of formula (3). Here, the generated gases can be cooled onand collected from the same surface to obtain a powder for a negativeelectrode material having a homogeneous distribution of a Liconcentration as in the aforementioned embodiment. Instead ofcontinuously heating the secondary raw material, the secondary rawmaterial may be subsequently reheated.

As described above, a raw material including elemental Si and lithiumsilicate (a Si.lithium silicate-containing raw material) can be obtainedby heating and calcining a primary raw material including elemental Siand LiOH. The resulting raw material obtained as a secondary rawmaterial can be heated to generate a SiO gas and a Li gassimultaneously.

Li₂CO₃ may also be used instead of LiOH. That is, a Si powder is mixedwith a Li₂CO₃ powder. A SiO₂ powder is mixed to adjust the O content ifnecessary. The resulting powder mixture as a primary raw material ischanged into a reaction vessel and heated and calcined under an Aratmosphere. With reference to chemical formulae, a reaction in whichLi₂CO₃ is heated in the co-presence of elemental Si may be expressed bythe first part of formula (4).

As shown in the first part of formula (4), heating and calcining Li₂CO₃in the co-presence of elemental Si generates lithium silicate (Li₄SiO₄)while an undesired element C is removed as a gas component. Theresulting calcined material is a mixture of lithium silicate (Li₄SiO₄)and residual element Si. This corresponds to the raw material containingSi, O, and Li used in the aforementioned embodiment.

Then, heating of the resulting calcined material as a secondary rawmaterial is continued under reduced pressure. Heating of lithiumsilicate (Li₄SiO₄) contained in the secondary raw material in theco-presence of elemental Si then generates a Si gas and a Li gassimultaneously from that lithium silicate (Li₄SiO₄) as shown in thelatter part of formula (4). Here, the generated gases can be cooled onand collected from the same surface to obtain a powder for a negativeelectrode material having a homogeneous distribution of a Liconcentration as in the aforementioned embodiment. Instead ofcontinuously heating the secondary raw material, the secondary rawmaterial may be subsequently reheated.

As described above, a raw material including elemental Si and lithiumsilicate (a Si.lithium silicate-containing raw material) can be obtainedby heating and calcining a primary raw material including elemental Siand Li₂CO₃. The resulting raw material as a secondary raw material canbe heated to generate a SiO gas and a Li gas simultaneously. LiOH andLi₂CO₃ may also be used instead of using LiOH or Li₂CO₃.

It is noted that the chemical reactions in the embodiments arerepresented by chemical formulae (1) to (4), but these merely representputative reactions in model cases in which these phenomena aresimplified. The actual reactions may likely be more complicated due tothe addition of SiO₂ for adjusting the O content.

EXAMPLE 1

A Li powder, a SiO₂ powder, and a Li₂Si₂O₅ powder were mixed in a molarratio of 21:15:2. The element ratio of the powder mixture isSi:Li:O=1:0.1:1. This powder as a raw material was charged into areaction vessel and heated to 1400° C. under reduced pressure. Generatedgases were cooled on and collected from a vapor deposition platformarranged at an upper portion within the reaction vessel. Then, thecollected material (deposit) was pulverized into a powder with a ballmill using a zirconia container and balls. The mean particle size of thepowder was 5.2 μm as determined by the laser diffraction particle sizedistribution measurement.

From the resulting powder, 10 particles were randomly selected for thecross-sectional TEM observation of each particle. EELS measurements wereperformed at a depth of 50 nm from the outmost surface of a particle fora region of 20 nm in the longitudinal direction and 400 nm in thetransverse direction to obtain a Si spectral intensity and a Li spectralintensity. The ratio of the Li spectral intensity to the Si spectralintensity was taken as L1, i.e., a Li concentration at the surface ofthe particle. A similar procedure was performed at a depth of 400 nmfrom the outmost surface of the particle to obtain the ratio of a Lispectral intensity to a Si spectral intensity, which was taken as L2,i.e., a Li concentration in the inside of the particle. L1/L2 wasdetermined for each of the 10 particles, and the standard deviation ofL2 was then determined.

For the measurements of a particle, the particle was cut to expose across-section under an inert atmosphere by the FIB process using anFB-2000A (Hitachi). The TEM observation was performed under an atomicresolution-analytical electron microscope JEM-ARM 200F (JEOL), and thenEELS measurements were performed with a GATAN GIF Quantum energy filter.The TEM measurements were performed under the following conditions: theacceleration voltage was 200 kV; the diameter of a beam was 0.2 nmφ; andthe energy resolution was 0.5 eV FWHM.

Further, the Li content (Li/Si) and the O content (O/Si) of the powderobtained were measured by the ICP emission analysis method and theinfrared absorption method.

EXAMPLE 2

A Si powder and a Li₂Si₂O₅ powder were mixed in a molar ratio of 3:1.The element ratio of the powder mixture is Si:Li:O=1:0.4:1. Otherprocedures were the same as Example 1, and a powder with an averageparticle size of 5.4 μm was produced. Then, L1/L2 and the standarddeviation of L2 of the resulting powder were determined as well as theLi content and the O content. FIG. 1 shows a cross-sectional TEM imageof a particle of the powder which was used for measuring L1 and L2 ofthat particle.

EXAMPLE 3

A Si powder and a Li₂SiO₃ powder were mixed in a molar ratio of 2:1. Theelement ratio of the powder mixture is Si:Li:O=1:0.67:1. Otherprocedures were the same as Example 1, and a powder with an averageparticle size of 5.1 μm was produced. Then, L1/L2 and the standarddeviation of L2 of the resulting powder were determined as well as theLi content and the O content.

EXAMPLE 4

A Si powder, a SiO₂ powder, and a LiOH powder were mixed in a molarratio of 4:1:3. The resulting powder as a primary raw material wascharged into a reactional vessel and heated and calcined to 1400° C.under an Ar atmosphere at the atmospheric pressure. A portion of thecalcined material was collected and analyzed. The element ratio wasfound to be Si:Li:O=1:0.6:1, and no residual H component was found.

Then, the calcined material was used as a secondary raw material, andallowed to be continuously heated at 1400° C. in the same reactionvessel under reduced pressure. Generated gases were then cooled on andcollected from a vapor deposition platform arranged at an upper portionwithin the reaction vessel. Subsequently, the collected material(deposit) was subjected to powderization as in Example 1, and then L1/L2and the standard deviation of L2 of the powder were determined as wellas the Li content and the O content. The mean particle size of theresulting powder was 5.6 μm.

COMPARATIVE EXAMPLE 1

A Si powder and a SiO₂ powder were mixed in a molar ratio of 1:1. Theelement ratio of the powder mixture is Si:Li:O=1:0:1. Other procedureswere the same as Example 1, and a powder with an average particle sizeof 5.1 μm was produced. The resulting powder did not contain Li, andthus only the O content was measured.

COMPARATIVE EXAMPLE 2

A powder of lithium hydride (LiH) was added to the powder produced inComparative Example 1, i.e., a SiO powder so that Li was present at 0.4mol relative to Si and O, and then the resulting powder was heated andcalcined to 850° C. under an Ar atmosphere to obtain a powder with amean particle size of 5.2 μm. Then, L1/L2 and the standard deviation ofL2 of the resulting powder were determined as well as the Li content andthe O content.

COMPARATIVE EXAMPLE 3

Two reactional vessels were provided. A mixture in which a Si powder anda SiO₂ powder were mixed in a molar ratio of 1:1 was charged into onevessel. The element ratio of the powder mixture is Si:Li:O=1:0:1.Further, metal Li was charged into the other vessel under an inert gasatmosphere. Then, the weight ratio and the heating temperature of theraw materials in the two vessels were adjusted so that a SiO gasgenerated in one vessel and a Li gas generated in the other vesselshowed a partial pressure of 1:0.4. The gases generated in both vesselswere mixed and cooled on and collected from the common vapor depositionplatform.

Subsequently, the collected material (deposit) was subjected topowderization as in Example 1, and L1/L2 and the standard deviation ofL2 of the resulting powder were determined as well as the Li content andthe O content. The mean particle size of the resulting powder was 5.2μm.

Battery Evaluation

Battery evaluation was performed according to the following procedurefor the powder samples produced in Examples 1 to 4 and ComparativeExamples 1 to 3.

A powder sample, a PI binder as a nonaqueous (organic) binder, and a KBas an electrically conductive auxiliary agent were mixed in a weightratio of 80:15:5 and kneaded with an organic solvent NMP to obtain aslurry. The resulting slurry was applied onto a copper foil andsubjected to vacuum heat treatment at 350° C. for 30 minutes to obtain anegative electrode. The resulting negative electrode, a counterelectrode (a Li foil), an electrolytic solution (EC:DEC=1:1), anelectrolyte (1 mol/L of LiPF), and a separator (a polyethylene porousfilm with a film thickness of 30 μm) were combined to fabricate a coincell battery.

The resulting coin cell battery was subjected to a charge and dischargetest. Charge was performed at a constant current of 0.1 C until thevoltage across the two electrodes of the battery reached 0.005 v. Afterthe voltage reached 0.005 v, constant-potential charge was thenperformed until the electric current reached 0.01 C. Discharge wasperformed at a constant current of 0.1 C until the voltage across thetwo electrodes of the battery reached 1.5 V.

The initial charging capacity and initial discharge capacity weremeasured by this charge and discharge test to determine the initialefficiency. Results are shown in Table 1 along with the mainspecifications (the Li content, the O content, L1/L2, and the standarddeviation of L2) of powder samples.

TABLE 1 Raw Initial Material Post-rxn Post-rxn L2 effi- Li/Si Li/Si O/SiL1/L2 Std. Dev. ciency Example 1 0.1 0.08 1.03 0.94-1.03 0.05 74.30%Example 2 0.4 0.41 0.99 0.95-1.06 0.04 79.10% Example 3 0.67 0.95 1.050.91-1.05 0.05 82.50% Example 4 0.6 0.93 1.02 0.88-1.15 0.07 82.20%Comp. 0 0 1.04 — — 71.40% Exp. 1 Comp. 0 0.4 1.08 1.19-1.57 0.13 68.60%Exp. 2 Comp. 0.4 0.34 0.98 0.90-1.11 0.16 29.70% Exp. 3

In Comparative Example 1, Li was not doped to a SiO powder. As comparedwith this, the initial efficiency was improved for all of Examples 1 to4, demonstrating that the performance was improved by Li doping.Incidentally, L1/L2 as a ratio of a Li concentration in the inside of aparticle to a Li concentration at the surface of the particle fallswithin a range of 0.8<L1/L2<1.2, and the standard deviation of L2 isalso 0.1 or less for all cases.

In contrast, in Comparative Example 2, Li was doped in accordance withthe solid phase method (calcination method) as described in PatentDocument 1. As often observed in this method, Li in a higherconcentration was unevenly distributed at the surface of a particle, andL1/L2 as a ratio of a Li concentration in the inside of a particle to aLi concentration at the surface of the particle showed significantvariation toward a value of greater than 1.2. This resulted indeteriorated binder performance and a decreased initial efficiency ascompared with even Comparative Example 1.

In contrast, in Comparative Example 3, Li was doped in accordance withthe gas phase method (deposition method) as described in Patent Document2. Use of a gas mixture of a SiO gas and a Li gas allowed for a smallerdifference in the Li concentration between the surface of a particle andthe inside of the particle, but the Li concentration showed significantvariation among particles, resulting in an even lower initial efficiencyas compared with Comparative Example 2. This is likely because particleswith higher Li concentrations were produced, and they reacted with abinder.

Incidentally, the Li content (x=Li/Si) and the O content (y=O/Si) in apowder satisfies 0.05<x<y<1.2 as defined in the present invention forall samples except for Comparative Example 1 where Li was not doped.This also indicates that L1/L2 and the standard deviation of L2 areeffective performance measures for a powder of a negative electrodematerial.

1. A silicon oxide-based negative electrode material, comprising apowder having an average composition of SiLi_(x)O_(y) wherein0.05<x<y<1.2 and a mean particle size of 1 μm or more, wherein 10particles randomly selected from particles of the powder each satisfy0.8<L1/L2<1.2 with the standard deviation of L2 being 0.1 or less, L1being a Li concentration at a depth of 50 nm from an outermost surfaceof each of the 10 particles, and L2 being a Li concentration at a depthof 400 nm from the outermost surface.
 2. A method of manufacturing asilicon oxide-based negative electrode material, the method comprising:heating a raw material containing Si, O, and Li to generate a SiO gasand a Li gas simultaneously from the raw material, and cooling the gaseson the same surface to allow for collection.
 3. The method ofmanufacturing a silicon oxide-based negative electrode materialaccording to claim 2, wherein a portion of Si contained in the rawmaterial is present as elemental Si, and Li is present as lithiumsilicate.
 4. The method of manufacturing a silicon oxide-based negativeelectrode material according to claim 3, wherein the raw material is amixture of elemental Si and lithium silicate, or a mixture of elementalSi, lithium silicate, and a Si oxide.
 5. The method of manufacturing asilicon oxide-based negative electrode material according to claim 4,comprising: heating and calcining a primary raw material including oneor both of LiOH and Li₂CO₃ in addition to elemental Si to obtain asecondary raw material, and heating the secondary raw material as theraw material.
 6. The method of manufacturing a silicon oxide-basednegative electrode material according to claim 2, wherein the averagecomposition of the raw material is represented by SiLi_(x)O_(y) wherein0.05<x<0.7 and 0.9<y<1.1.